Preheat your oven to 350 degrees F. Line a baking sheet (or two) with parchment paper to prevent sticking.
Take half of the old-fashioned rolled oats and place them in a food processor. Pulse until they are chopped up, resembling a coarse flour, but be sure to leave some larger pieces for texture.
In a large bowl, combine the processed oats with the remaining old-fashioned rolled oats, all-purpose flour, baking soda, cinnamon, cardamom, nutmeg, and salt. Whisk everything together until well mixed.
In another large bowl, whisk the melted and cooled unsalted butter with the brown sugar until the mixture is smooth and creamy.
Add in the egg, egg yolk, and vanilla extract, whisking until fully combined.
Using a large spoon, gradually stir in the dry ingredients from your first bowl. Mix until a dough forms, making sure not to overmix.
Scoop about 2 tablespoons of dough using an ice cream scoop and place them on the prepared parchment paper, spaced about 2 inches apart. The cookies will spread as they bake, so leave enough room!
Pop the baking sheet into the oven and bake for 12 to 14 minutes, or until the edges are golden brown and the center remains soft and chewy.
Once baked, remove the cookies from the oven and let them cool completely on the baking sheet.
While the cookies cool, prepare the brown butter icing. In a small saucepan over medium-low heat, melt unsalted butter. Continue to whisk as it melts, watching for it to bubble and foam. Eventually, brown bits will form at the bottom; this is what you're aiming for and should take about 5-6 minutes. Once browned, remove from heat and let it cool for about 10 minutes.
In a large bowl, combine the cooled brown butter, vanilla extract, and powdered sugar. Whisk until the mixture comes together, then stir in 1 tablespoon of milk. If your glaze seems too thick, add more milk, a half tablespoon at a time, until you achieve your desired consistency.
Finally, drizzle the brown butter icing over the cooled cookies and let it set before serving. Enjoy your Thin and Chewy Spiced Oatmeal Cookies with family and friends!
